The use of secondary power systems, such as auxiliary power units ("APUs"), is well known in aerospace technology. A primary function of an APU is to provide electrical power, hydraulic power, pressurized air, or other power when the main engines are unable to do so, such as when the aircraft is on the ground. The development of high-performance aerodynamically unstable aircraft has underscored the necessity of a rapidly available onboard source of emergency power such as that available from an APU. Upon a failure of the main hydraulic pump supplying power to aircraft control surfaces, the main generator supplying electrical power to flight control computers or the main aircraft engine driving such devices, the aircraft cannot be maintained in controlled flight. Because an APU is an air-breathing engine requiring a minimum ambient pressure in order to function, a conventional APU is not capable of operation above a certain altitude. Additionally, should the aircraft go into a spin or other anomalous flight condition, the APU may not properly function due to separated or other irregular airflow at the air inlet to the APU. Accordingly, an aircraft with only an APU may require some other system, such as an emergency power unit ("EPU") well known in the art, to provide power to the aircraft until the APU or the main engine can be re-started.
In addition to conventional propulsion functions, aircraft main engines are often called upon to provide pressurized air for the environmental control system ("ECS"), which uses such air to cool or heat, ventilate and pressurize the aircraft cabin and to provide cooling to the avionics.
Equipping an aircraft with an APU, EPU and ECS presents several problems to aircraft designers and operators, among which are an increase in required onboard space, costly equipment redundancies and additional weight. Several approaches to this problem have been developed, primarily directed to integrating the APU and EPU or the APU and ECS into one power unit. However, such approaches retain the limitations associated with a conventional APU in that these integrated systems may be inoperable at high altitude or during other flight conditions where air breathing operation is not possible.
Accordingly, a need exists for an integrated aircraft subsystem that provides the functions of an APU, EPU and ECS while not being subject to altitude and other flight condition constraints.